Cardiac dysrhythmias are commonly known as irregular heart beats or racing heart. Two such heart rhythm irregularities are the Wolff-Parkinson-White syndrome and atrioventricular (“AV”) nodal reentrant tachycardia. These conditions are caused by an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses normally existing in the heart. For example, in one type of the Wolff-Parkinson-White syndrome the accessory pathway causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber. Another common type of cardiac dysrhythmias is ventricular tachycardia (“VT”), which is a complication of a heart attack or reduction of blood supply to an area of heart muscle, and is a life threatening arrhythmia. An even more common type of cardiac dysrhythmias is Atrial Fibrillation which afflicts millions of people worldwide.
In the treatment of cardiac dysrhythmias, non-surgical procedures such as management with drugs are favored. However, some dysrhythmias of the heart are not treatable with drugs. These patients are then treated with either surgical resection of VT site of origin or by Automatic implantable cardiovertor defibrillator (“AICD”). Both procedures have increased morbidity and mortality and are extremely expensive. Even AICD needs major surgical intervention. In addition, some patients of advanced age or illness cannot tolerate invasive surgery to excise tachycardia focus which causes dysrhythmias.
Techniques have been developed to locate regions of tachycardia and to disable their short-circuit function. Radio-frequency energy is applied to ablate the cardiac tissues in those regions so as to produce scars and interrupt conduction.
The regions to be ablated are usually determined by endocardiac mapping. It is a technique that typically involves percutaneously introducing a mapping electrode catheter into the patient. The mapping electrode catheter is passed through a blood vessel, like femoral vein or aorta and thence into an endocardiac site such as the atrium or ventricle of the heart. A tachycardia is induced and a continuous, simultaneous recording made with a multichannel recorder while the electrode catheter is moved to different endocardiac positions. When a tachycardial focus is located as indicated in an electrocardiogram recording, it is marked by means of a fluoroscope image.
Upon locating of the tachycardial focus, ablation of cardiac arrhythmias is typically performed by a standard ablating electrode catheter placed at the focus. The Radio-frequency energy is used to create a lesion in the endocardiac tissues adjacent (i.e. underneath) the standard electrode catheter. By creating one or more lesions, the tachy ardial focus may be turned into a region of necrotic tissue, thereby disabling any malfunctions.
Conventional catheter ablation techniques have typically employed a catheter with a single electrode at its tip as one electrical pole. The other electrical pole is formed by a backplate in contact with a patient's external body part. These techniques have been used successfully for interruption or modification of conduction across the atrioventricular (AV) junction in AV nodal reentrant tachycardia; for interruption of accessory pathway in patients with reentrant tachycardia due to Wolff-Parkinson-White Syndrome; and for ablation in some patients with ventricular tachycardia.
In one technique, high voltage direct current (“DC”) in the range of 100 to 300 joules is applied across the electrode and the backplate to effect ablation. Direct current energy source using the standard electrode catheter can produce a lesion size larger than the footprint of the electrode. However, the lesion dimensions are variable at the same energy output and they do not have clear demarcation from the surrounding tissues. Additionally, high voltage techniques have other undesirable side-effects such as barotrauma and the lesions formed could become proarrhythmic. This technique is now abandoned.
Another technique is to apply a radio-frequency (“RF”) source to a standard electrode catheter. The RF source is typically in the 600 kHz region and produces a sinusoidal voltage between two wires. When this is delivered between the distal tip of a standard electrode catheter and a backplate, it produces a localized RF heating effect. It causes a well defined, discrete lesion slightly larger than the tip electrode. This simple RF ablation technique creates lesion size sufficient for interruption of AV junction or accessory pathway.
RF ablation is preferable to DC ablation because it does not need anesthesia and produces more circumscribed and discrete lesions and avoids injury caused by high voltages as in DC shock.
Generally, catheter ablations of AV junction using standard electrode catheter with DC or RF energy for treating drug resistant supraventricular tachycardia have high success rate with very low incidence of complications. For Cardiac arrhythmias like Superaventricular tachycardia (“SVT”), Idiopathic ventricular tachycardia, Ischemic ventricular tachycardia and more recently Atrial fibrillation Radiofrequency catheter ablation has become principal form of therapy. In 50% of VT and 10% of SVT deeper lesions may be needed and standard 7 f 4 mm catheter electrode may be unable to create deeper lesion to ablate arrhythmogenic substrate.
However, in ventricular tachycardia (VT), endocardiac mapping with a standard electrode catheter can locate the exit site of ventricular tachycardia to within 4 to 8 cm2 of the earliest site recorded by the catheter. A standard electrode catheter typically has a maximum electrode tip area of about 0.3 cm2. Therefore, the lesion created by the simple RF technique delivered through a standard electrode catheter may not be large enough to ablate the ventricular tachycardia. Attempts to increase the size of lesion by regulation of power and duration by increasing the size of electrode or by regulating the temperature of tip electrode have met with partial success.
In order to increase the size of the lesion, an orthogonal electrode catheter array (OECA) with four peripheral electrodes and one central electrode has been proposed. Such an OECA has been disclosed by Dr. Jawahar Desai in U.S. Pat. No. 4,940,064, issued Jul. 10, 1990, for use in both mapping and ablation of endocardiac sites.
In spite of the improvements, the need remains for further improvements in creating lesions of desirable size in a minimum of time with minimum undesirable side effects.
It is generally recognized that lesions of larger and deeper size are achieved by increasing the input RF power. One problem has to do with overheating which can cause the ablation system to malfunction and other dangerous side effects, such as the formation of blood clot in the course of RF ablation. Experimental data suggest a lesion is created when myocardial tissue is irreversibly damaged at temperature higher than 50° C. Deeper lesions are produced as catheter tip-tissue interface temperature increases, until the interface temperature reaches 100° C., at which point plasma boils, resulting in coagulum formation at the electrode surface. This can result in clot embolization, a sudden increase in impedance of the ablation circuit, resulting in ineffective tissue heating. More seriously, the blood clots may block blood vessels such as those in the brain and result in the patient suffering a stroke.
Placement of thermocouples and thermistors at the catheter tip allows monitoring of catheter tip temperature, in an attempt to avoid excessive heating. Subsequent RF generators have allowed titration of delivered power until a chosen catheter tip temperature is reached. RF delivery in this mode is referred to as temperature-guided RF ablation. However, this technique necessarily results in a longer ablation time and causes complications that accompany a prolonged procedure.
In order to supply more power without excessive heating, the ablation electrode is cooled to keep the temperature under control. Since blood is at a temperature of about 37 degree centigrade, the electrode is designed to have a large surface area in contact with blood which serves to cool the electrode. The cooling by blood is effective especially in the heart chamber where substantially volume of it is constantly being exchanged.
In situations with low blood flow, the electrode is additionally cooled by irrigation with a coolant. As pointed out in Wittkampf et al, “RF Catheter Ablation: Lessons on Lesions”, PACE, Vol. 29, November 2006, pp. 1285-1297, low blood flow may occur in atrial fibrillation or poor left ventricular function. This will result in limited cooling of the electrode and impose a limitation on the amount of power that can safely be applied. Extraneously supplied coolant is used to augment the cooling of the electrode.
The above mentioned techniques help to alleviate some of the problems but also create other undesirable effects such as inefficient power usage, generating a substantial amount of heat from wasted power, long ablation time, excessive amount of coolant introduced into the patient, and still do not eliminate the danger of blood clot formation.
Thus, it is desirable to have catheter ablations that produce lesions of desirable size while performing them more efficiently in less time with less power and coolant and less danger of blood clot formation.